1. Field of the Invention
The present invention relates to a heat-transfer tube with a grooved inner surface and, more particularly, to an improved inner surface grooved heat-transfer tube adapted to phase-transition of fluid flowing inside the tube disposed in a heat exchanger such as an air conditioner, refrigerator, boiler, etc.
The inner surface grooved heat-transfer tube (hereinafter called "inner surface grooved tube") has a number of spiral grooves on an inner surface of a metal tube such as a copper tube or the like, as shown in FIG. 1.
While this type of conventional inner surface grooved tubes improved by limiting the depth, shape and helix angle of the grooves, etc. have been disclosed, they do not sufficiently meet the requirements of users. The maximal reason for it is due to the low ratio of heat-transfer characteristic to manufacturing cost of the tube. That is, because the inner surface grooved tube has an inner surface of fine and irregular structure, it is difficult to provide the stable quality to the tube unless utilizing a rolling process. However, the rolling process has the limitation in production speed based on the revolution rate of a motor and the like, in other words, the limitation of manufacturing cost. On the other hand, a groove free tube can be made by a high speed drawing process. Therefore, considering the conventional inner surface grooved tube based on the ratio of the heat-transfer characteristic to the manufacturing cost, it is not easy to provide the switchover merit of the groove free tube to the grooved tube.
The configurations or shapes of the conventional typical inner surface grooved tubes are shown in FIGS. 2(a) and 2(b). There conventional grooved tubes have a low ratio of the characteristics to the manufacturing cost due to the following two reasons:
(1) It is well known that the characteristic or performance is proportional to the depth (Hf) of the grooves. The limit which the pressure loss in the grooved tube increases sharply, compared with the groove free tube exists in the vicinity of 0.02 to 0.03 (this value is represented by the ratio of the depth (Hf) of the groove to the inside diameter (Di) of the tube). The conventional grooved tube has nevertheless a value, Hf/Di, of less than about 0.018 and therefore, the groove depth of the conventional tube does not reach the above mentioned optimum limit. This is also attributable to the reasons that the increase of the groove depth in the conventional tube is related to the weight per unit length of the tube and thus, a higher cost.
(2) The factors affecting the characteristics of the tube are the shapes of groove and ridge formed on the inner surface. The conventional product shown in FIG. 2 (a) has insufficient characteristics because the cross-sectional area (S) of the grooved section is small and the helix angle (.alpha.) of the ridge is large. Although the cross-sectional area (S) of the product shown in FIG. 2(b) is larger than that of 2(a), it has insufficient characteristics due to its trapezoidal ridge.